Factors influencing Pi transport in the kidney have been extensively studied during these last years: phosphorus depletion and loading, Na transport, pH, vitamin D, hormones. The early steps of PTH action upon Pi transport has also been carefully investigated by the technique of luminal and antiluminal membranes separation. But the terminal step of these sequencial events, the Pi transport through the luminal membrane itself remains obscure. It is generally thought to be an active transport, facilitated by alkaline phosphatase and on the contrary, curtailed by the cAMP stimulated phosphorylation of the membrane. The purpose of this study is to measure the alkaline and acid phosphatase activity along the different segments of the nephron, using the microdissection technique we recently learned in Morel's laboratory. We want to know if the sites of high phosphatase activity correspond to the sites of Pi transport in the mice. We shall also measure the phosphatase activity along the nephron in various experimental conditions, particularly those concerning the hormones which modify the phosphorus transport, and also in genetic hypophosphatemia.